Electronically commutated motors are used in a wide variety of applications, and particularly with power tools such as drills, saws, sanders, etc. Such motors are used with cordless power tools that are powered from a rechargeable DC battery. With cordless power tools, a wide variety of tasks often need to be performed that require different motor performance characteristics to best perform the work task. For example, when drilling a hole with a cordless drill in a soft piece of wood, the amount of torque required, and thus the power output required from the motor, may be only a small fraction of what would be needed to drill through a piece of hardwood or pressure treated lumber. However, designing the motor for high power applications is inefficient, from a power standpoint, if the same drill will frequently be used with tasks that involve only light duty drilling, where only low torque is needed for the work task. Accordingly, a motor designed to provide high power will draw additional battery current that may not be needed for many drilling tasks. This will reduce the run time of the battery powering the tool, compared to the run time that could be achieved with a motor designed for a lower maximum power output.
Accordingly, it would be beneficial to provide a motor having a plurality of distinctly different operating modes that provide varying degrees of motor speed, torque and power output, to better match the needs of specific work tasks. For example, it would be highly beneficial if a motor and associated control system was provided that could automatically sense when additional motor power is required when performing a given task, and the motor automatically switched to a specific operating mode to either increase or decrease the torque and/or operating speed of the motor. Alternatively, it would be desirable if the different operating modes of the motor could be selected by a user via a control on the power tool. This would enable the optimum operating characteristics of the motor to be employed, and changed, as needed for different work tasks. Optimizing the motor performance would also lead to the most efficient use of available battery power. This could serve to significantly extend the run time of the battery on a given charge.
Still another factor in optimizing the performance of an electronically commutated motor is the construction of the rotor used in such a motor. The typical construction of such a rotor can be broadly classified as either “surface mounted” or “interior mounted”. With a surface mounted construction, the permanent magnets of the rotor are secured to the outer surface of a rotor back iron. With an interior mounted construction, the permanent magnets are typically rectangular in shape and secured in pockets or recesses formed in the rotor back iron. The surface mounted rotor construction is not as well suited for use with motors that will be operated over a wide operating range of speeds. At high operating speeds, keeping the magnets attached to the outer surface of the rotor back iron can present challenges. However, the surface mounted magnet construction can be implemented with a relatively simple electronic controller that controls energization of the windings on the stator. The surface mounted magnet construction also provides higher flux output, and thus higher power output for a given size of motor in which a rotor with surface mounted magnets is implemented.
With the internally mounted magnet construction, the problem of maintaining the magnets attached to the rotor back iron is alleviated, and the rotor is well suited for use in motors having a wide range of operating speeds. However, the internally mounted magnet construction requires a relatively complex controller to be used to compensate for the difference in the magnetic “gap” between the direct axis and the quadrature axis of each magnet. The interior mounted magnets also do not generate the same degree of flux output, for a given size motor, as surface mounted magnets, which will contribute to a lower power output for an electronically commutated motor in which the rotor is used.
As a result, it would also be desirable to provide a rotor for an electronically commutated motor that provides the performance benefits of surface mounted permanent magnet construction, without the construction drawbacks associated with this type of construction.
Still further, in power tool applications, it would be desirable if the motor could be controlled to better match its speed-torque performance curve to that of the gear system being used with the tool. With power tools such as grinders, drills, saws, etc., the gear system coupled to the output of the motor is typically made sufficiently robust so that it's speed-torque performance characteristics are a predetermined degree greater than those of the motor. This is to ensure that the speed-torque performance capability of the motor can be readily handled by the gear system without risking failure or unacceptable stress to the gear system. If the motor speed-torque performance characteristics could be closely matched to those of the gear system via electronic control over the motor, without exceeding the speed-torque performance characteristics of the gear system, then optimum use of the gear system could be made. Alternatively, this would allow a less expensive, and less robust gear system to be employed in connection with a given motor, because the speed-torque performance “headroom” normally required to be designed into the gear system would not be needed.